been persuaded to leave intervals of 3 to 6 years between births, the prematurity rate for subsequent pregnancies would have been reduced by 21 percent. (67, p. 158)
It should be added that other factors may have been operating in the longer interval group. These women may have had abnormalities which accounted for infertility and prematurity.
Birth Interval and Growth. In the studies reported from Candelaria (10), growth was used as the indicator of nutritional status and was found to be associated with birth interval. One other study, that of Grant (37), also examined the effect of birth interval. She, too, was concerned about the effect of interval on the preceding child. Her conclusions regarding the cross-effects of birth rank and family size may be recalled here: ". . . the advent of each additional child to a family acts as a check on the growth of all preceding sibs. . . ." (37, p. 38).
Her findings for birth interval and growth are presented in Table 18. The differences support the view that birth interval has an effect, but she notes that they are not statistically significant.
Mean Height at Sixth Birthday Related to Interval between Births, London, 1953-1960
Number of Children in Family Mean Height (cm.)
2 Mean for all "first of two" Mean for 1st when 2nd follows within 2 years 114.0 113.0 113.9 113.1
3 Mean for all "first of three" Mean for 1st when 2nd follows within 2 years 115.6 116.5 115.0 113.0
Mean for all "second of three" Mean for 2nd when 3rd follows within 2 years 114.6 113.2 112.0 109.6
Source: Grant (37).
Birth Interval and Intelligence. We saw abundant evidence of a negative correlation between intelligence test performance (on a wide variety of such tests) and family si/.e. There is no such abundance when it comes to evidence concerning the effects of birth interval. In fact, it appears that only in the British National Survey of Health and Development has the matter been examined. Douglas and his colleagues, who reported the findings in this partshe first reported it in 1935 (65). In discussing